1. Field of the Invention
The present invention relates to an acoustic matching member used for an acoustic matching layer of an ultrasonic sensor, an ultrasonic transducer for transmitting/receiving ultrasonic waves, a method for manufacturing them, and an ultrasonic flowmeter using them.
2. Related Background Art
In recent years, an ultrasonic flowmeter has been used as a gas meter and the like, where a time for ultrasonic waves to propagate through a propagation path and a velocity of fluid moving therein are measured so as to determine a flow rate of the fluid. FIG. 13 shows the principles of measurement by the ultrasonic flowmeter. As shown in FIG. 13, within a measurement tube including a flow path, fluid flows at a velocity of V in the direction shown by the arrow in the drawing. In a tube wall 103, a pair of ultrasonic transducers 101 and 102 is disposed so as to oppose each other. The ultrasonic transducers 101 and 102 are configured with a piezoelectric vibrator such as a piezoelectric ceramic functioning as an electric/mechanical energy transducer, and therefore exhibit resonant characteristics like a piezobuzzer and a piezoelectric oscillator. In this case, the ultrasonic transducer 101 is used as an ultrasonic transmitter and the ultrasonic transducer 102 is used as an ultrasonic receiver.
These ultrasonic transducers operate as follows: when an AC voltage at a frequency dose to a resonant frequency of the ultrasonic transducer 101 is applied to the piezoelectric vibrator, the ultrasonic transducer 101 operates as an ultrasonic transmitter so as to emit ultrasonic waves to a propagation path in the fluid flowing in the tube, which is indicated by L1 in the drawing, and the ultrasonic transducer 102 receives the ultrasonic waves that have propagated and converts them to voltage. Subsequently, the ultrasonic transducer 102 conversely is used as an ultrasonic transmitter and the ultrasonic transducer 101 is used as an ultrasonic receiver. That is, by applying an AC voltage at a frequency dose to a resonant frequency of the ultrasonic transducer 102 to the piezoelectric vibrator, the ultrasonic transducer 102 emits ultrasonic waves to a propagation path in the fluid flowing in the tube, which is indicated by L2 in the drawing, and the ultrasonic transducer 101 receives the ultrasonic waves that have propagated and converts them to voltage. In this way, each of the ultrasonic transducers 101 and 102 serves as the receiver and the transmitter, and therefore, in general, they are called an ultrasonic transmitter/receiver.
In such an ultrasonic flowmeter, the continuous application of an AC voltage results in the continuous emission of ultrasonic waves from the ultrasonic transducer, which makes it difficult to measure the propagation time. Therefore, normally, a burst voltage signal is used as a driving voltage, where a pulse signal is used as a carrier wave. A more detailed description of the measurement principles will be given below. By applying a burst voltage signal to drive the ultrasonic transducer 101 and allow the ultrasonic transducer 101 to emit an ultrasonic burst signal, this ultrasonic burst signal propagates through a propagation path L1 with a length of L to arrive at the ultrasonic transducer 102 after the time t has elapsed. The ultrasonic transducer 102 can convert the ultrasonic burst signal that has propagated only into an electric burst signal at a high S/N ratio. This electric burst signal is amplified electrically and is applied again to the ultrasonic transducer 101 to allow an ultrasonic burst signal to be emitted. This device is called a sing around device. A time required for an ultrasonic pulse to be emitted from the ultrasonic transducer 101 and propagate through the propagation path to arrive at the ultrasonic transducer 102 is called a sing around period, and the reciprocal of the sing around period is called a sing around frequency.
In FIG. 13, V denotes a flow velocity of fluid that flows through the tube, C (not illustrated) denotes a velocity of an ultrasonic wave in the fluid and θ denotes an angle between the flowing direction of the fluid and the propagation direction of the ultrasonic pulse. When the ultrasonic transducer 101 is used as an ultrasonic transmitter and the ultrasonic transducer 102 is used as an ultrasonic receiver, the following formula (1) will be satisfied, where t1 denotes a sing around period that is a time for an ultrasonic pulse emitted from the ultrasonic transducer 101 to arrive at the ultrasonic transducer 102, and f1 denotes a sing around frequency:f1=1/t1=(C+Vcos θ)/L  (1)
Conversely, when the ultrasonic transducer 102 is used as an ultrasonic transmitter and the ultrasonic transducer 101 is used as an ultrasonic receiver, the following formula (2) will be satisfied, where t2 denotes a sing around period and f2 denotes a sing around frequency:f2=1/t2=(C−Vcos θ)/L  (2)
Therefore, a frequency difference Δf between the both sing around frequencies will be the following formula (3), so that the flow velocity V of the fluid can be determined from the length L of the propagation path of ultrasonic waves and the frequency difference Δf:Δf=f1−f2=2Vcos θ/L  (3)
That is to say, the flow velocity V of the fluid can be determined from the length L of the propagation path of ultrasonic waves and the frequency difference Δf, and a flow rate can be determined from the velocity V.
Such an ultrasonic flowmeter requires high accuracy. In order to improve the accuracy, an acoustic impedance of an acoustic matching layer becomes important, where the acoustic matching layer is formed on a surface for transmitting/receiving ultrasonic waves of the piezoelectric vibrator constituting the ultrasonic transducer for transmitting the ultrasonic waves to gas or receiving the ultrasonic waves that have propagated through gas.
FIG. 12 is a cross-sectional view showing a configuration of a conventional ultrasonic transducer 20. Reference numeral 10 denotes an acoustic matching layer functioning as an acoustic matching device, 5 denotes a sensor case, 4 denotes electrodes, and 3 denotes a piezoelectric member functioning as a vibration device. The sensor case 5 and the acoustic matching layer 10 or the sensor case 5 and the piezoelectric member 3 are bonded with an epoxy adhesive and the like. Reference numeral 7 of FIG. 12 denotes driving terminals, which are respectively connected to the electrodes 4 of the piezoelectric member 3. Reference numeral 6 denotes an insulation seal for securing electrical insulation of the two driving terminals. Ultrasonic waves generated from vibrations of the piezoelectric member 3 oscillate at a specific frequency, and the oscillation is conveyed to the case via the epoxy adhesive, and further is conveyed to the acoustic matching layer 10 via the epoxy adhesive. The matched oscillation propagates as an acoustic wave through gas as a medium that is present in the space.
This acoustic matching layer 10 has a role of allowing the vibrations of the vibration device to propagate effectively through the gas. The acoustic impedance Z will be defined as the following formula (4) using a sound velocity C and a density ρ of the substance:Z=ρ×C  (4)
The acoustic impedance is different significantly between the piezoelectric member as the vibration device and the gas as a medium to which ultrasonic waves are emitted (hereinafter called “emission medium”). For instance, the acoustic impedance of a piezo-ceramic such as PZT (lead zirconate titanate), which is a common piezoelectric member, is about 30×106 kg/m2/s. Whereas, for the gas as the emission medium, the acoustic impedance (Z3) of air, for example, is about 400 kg/m2/s. On a boundary surface between the substances with the thus different acoustic impedances, reflection occurs in the propagation of acoustic waves, so that the strength of the acoustic waves that have passed through there becomes weak. As a method for solving this, a substance is inserted between the piezoelectric member as the vibration device and the gas as the emission medium of ultrasonic waves, where the acoustic impedance of the inserted substance has a relationship shown by the formula (5) with the acoustic impedances Z0 and Z3 of the piezoelectric member and the gas, which is a commonly known method for improving the strength of the acoustic waves that pass through by alleviating the reflection of the sounds:Z=(Z0×Z3)(1/2)  (5)
The optimum value satisfying this condition where the acoustic impedances are matched becomes about 11×104 kg/m2/s. Substances that satisfy this acoustic impedance are required to be a solid having a small density and a low velocity of sound, as is understood from the formula (4). A material used generally is obtained by encapsulating a glass balloon or a plastic balloon in a resin material, which is then formed on a surface of an ultrasonic vibrator made of a piezoelectric member. In addition, a method of applying thermal compression to hollow glass beads, a method of allowing a molten material to foam and the like are used. These methods are disclosed by, for example, JP 2559144 B.
The acoustic impedances of these materials, however, are larger than 50×104 kg/m2/s, and a material having a smaller acoustic impedance is necessary for matching with a gas to obtain high sensitivity.
The above-described acoustic matching layer is not limited to a single layer, and it is generally and widely known that the acoustic matching layer preferably is configured with a plurality of layers of materials having different acoustic impedances so that their acoustic impedances are varied gradually between the acoustic impedances of the piezoelectric member as the vibration device and the gas as the emission medium of ultrasonic waves.
It is widely known that to laminate a plurality of acoustic matching layers each having a thickness adjusted to be about ¼ of the emission wavelength of the ultrasonic waves that pass through the acoustic matching layer, where the plurality of layers have different acoustic impedances, is effective for widening a band of the ultrasonic transducer. Preferably, the plurality of matching layers are configured so that their acoustic impedances decreases gradually from the acoustic impedance Z0 of the piezoelectric member to the acoustic impedance Z3 of the gas as the emission medium (Z0>Z3) (See for example “ultrasonic waves handbook” published by Maruzen, Aug. 30, 1999, page 108 and page 115). For example, as shown in FIG. 14A, it can be considered that the density in the acoustic matching layer 10 on the side of the piezoelectric member 3 is increased, whereas that on the side of the gas as the emission medium is decreased.
From the viewpoint of the principles, the acoustic matching layer may be configured with a plurality of layers. However, from the industrial viewpoint, an acoustic matching layer having a double layer structure is effective. That is to say, when consideration is given to the effect from the acoustic matching layer made up of a plurality of layers and an increase in the cost associated with the configuration, the acoustic matching layer having a double layer structure is effective. As an example of the acoustic matching layer configured with two different layers, JP 61(1986)-169100 A, for example, discloses the following: a laminated polymeric porous film is adhered to an ultrasonic wave emission surface of a first matching layer with a low density obtained by solidifying a minute hollow material to form a double layer structure, whereby the acoustic impedance matching can be performed effectively, and at the same time the sensitivity of the ultrasonic transducer can be improved.
In the case of the acoustic matching layer having a double layer structure, as shown in FIG. 14B, an ideal way is to arrange a matching member 11 with a relatively high density as a first layer on the side of the piezoelectric member 3 and arrange a matching member 12 with a relatively low density as a second layer on the side of the gas and to integrate these layers.
As described above, it is known that the acoustic matching layer configured with a plurality of members having different acoustic impedances, especially with two different members (layers), is effective in terms of the principles. However, there are not so many applications of such a configuration.
The inventors of the present invention have conducted a detailed study of the conventional acoustic matching members made up of a plurality of different members. As a result, it was found that the conventional members have the following three problems:
The conventional acoustic matching members often are manufactured by preparing different materials individually and by attaching them or a similar method (e.g., to apply a coating onto a surface). As a result, (1) the bonding face between the layers is weak physically, and therefore delamination becomes likely to occur during transmission and reception of ultrasonic waves due to the vibration, which causes malfunctions of the acoustic matching member and of an ultrasonic transducer and an ultrasonic flowmeter using the same. (2) When attaching different members with a third member such as an adhesive, the acoustic matching member assumes a three layer structure practically. Therefore, it becomes difficult to design the acoustic matching layer optimally. That is to say, the physical properties (density and velocity of sound) of the bonding material as an intermediate layer and the shape after bonding (thickness of the intermediate layer) cannot be ignored, so that the design becomes difficult. Even when the design can be done, the problems of limited options for bonding materials and complicated control of the thickness of the intermediate layer cannot be avoided. (3) The complicated manufacturing method in which different members are prepared individually and are attached results in an increase in the manufacturing cost of the ultrasonic transducer and of an ultrasonic flowmeter.
Especially, when a porous member as the low density member is selected for the attached acoustic matching member on the above-stated grounds of the principles, the bonded surface is not a flat face but many voids are present, which means that the practically effective bonding area is significantly small. Since the adhesion properties decrease with decreases in effective bonding area, the above problem (1) becomes more pronounced.
In addition, even when the bonding can be done, the bonding material used tends to penetrate to the porous member, so that, as shown in FIG. 15, an intermediate layer 13 as a locally formed high density portion would be generated at a portion to which the adhesive penetrates. Since this intermediate layer 13 is generated from the impregnation of voids of the porous member with the adhesive, this layer necessarily has a higher density than the first layer 11 and the second layer 12. As a result, the configuration deviates from the above-stated ideal configuration “to configure with a plurality of matching layers so that their acoustic impedances decreases gradually from the acoustic impedance Z0 of the piezoelectric member to the acoustic impedance Z3 of the gas as the emission medium (Z0>Z3)”, thus making the above problem (2) more pronounced. Also in the case where a liquid state material is applied to a porous member as the first layer, followed by drying and curing so as to form the second layer, the generation of an intermediate layer formed by the porous member impregnated with the liquid state material cannot be avoided, and therefore the similar problems would occur. In either case, the above-stated problems (1) and (2) become more pronounced.